The present invention is directed to a fly ash composition, and more particularly, to a fly ash composition which has a reduced susceptibility to alkali-silica reactivity when used in cementitious compositions.
Fly ash comprises finely divided inorganic products produced by the combustion of coal. Enormous amounts of fly ash are produced annually nationwide, principally from burning coal in electric power plants. Disposal of fly ash poses an increasingly difficult problem because the volume, the particulate nature and the varied chemical composition of fly ash limit the number of acceptable disposal sites. Such sites require sophisticated and expensive engineering, design, construction and operational controls to be in place to manage and dispose of the fly ash. Furthermore, the capacity of these disposal sites is not unlimited.
In recent years, fly ash has been proposed for use in cement compositions. For example, U.S. Pat. No. 4,382,649 to Heitzmann et al. describes a cement composition which contains fly ash. The composition contains from 50 parts to about 80 parts portland cement; from 13 parts to about 35 parts fly ash; and from 1 part to about 5 parts potassium carbonate. The composition may additionally include up to about 10 parts metakaolin; up to about 6 parts slag; and up to 4 parts of an admixture. However, such a composition may be subject to alkali silica reactivity.
Chemical reactions between alkalies and aggregates in concrete mixtures result in a shorter useful life and deterioration of structures formed from the concrete mixture. The alkali compounds in the mixture react with certain aggregates in the concrete, resulting in an increased pH of the pore solution in the concrete. For such reactions to take place, the concrete must be subjected to a certain amount of moisture during service.
Concrete is essentially a two-component system comprising a paste portion and an aggregate portion. The paste portion is a product of the hydration reaction between the cementitious materials and water. The paste portion is formed from a matrix of solid material with an internal network of interconnected pores. When water enters these pores, it moves through the pores, solubilizing salts of calcium and alkali metals. As these salts are dissolved, the pH of the pore solution increases as a result of an increase of hydroxide ions in the pores. As the hydroxide ions react with reactive silica in the aggregate to solubilize the silica, a gel is produced which may cause the concrete to crack and/or deterioriate. Researchers believe that this cracking is a result of the gel absorbing water.
Both Class F and Class C fly ash have been proposed for a partial replacement of cement in concrete mixes in order to reduce alkali-silica reactivity. However, it would be desirable to be able to replace larger amounts of cement to provide greater protection from alkali-silica reactivity.
Thus, a need has developed in the art for a fly ash composition which is less susceptible to alkali silica reactivity when used to form a concrete mix.
The present invention meets that need by providing a fly ash composition including a blend of Class C fly ash, potassium carbonate, and lithium carbonate which results in reduced alkali-silica reactivity of concrete formed using the fly ash composition. The fly ash composition of the present invention can be used in cementitious and concrete compositions and may be formulated to have a wide range of curing times so that it can be used for a variety of purposes from patching to making concrete objects.
According to one aspect of the present invention, a fly ash composition for use in a concrete mix is provided comprising from about 65 wt. % to about 99% wt. % Class C fly ash; from about 0.1 wt. % to about 4 wt. % of lithium carbonate, from about 0.1 to about 5 wt % potassium carbonate, and from about 0.1 to about 26% by weight of a retarding agent. The retarding agent is included in the composition to aid in controlling the rate of set of the resulting cement/concrete mix and is preferably selected from the group consisting of borax, boric acid, citric acid, metakaolins, and blends thereof. In one embodiment, the retarding agent comprises from about 0.1 to 10% by weight metakaolin. In an alternative embodiment, the retarding agent comprises from about 0.1 to 4 wt % Borax, from about 0.1 to 4 wt % boric acid, from about 0.1 to 8 wt % citric acid, and from about 0.1 to 10 wt % by weight metakaolin.
The fly ash composition preferably further includes from about 0.1 to 5 wt % of a chelating agent to enhance the strength of the resulting cement mix.
In another embodiment of the invention, a concrete mix is provided comprising stone, sand, water, and Portland cement, where the mix includes from about 25 to 40 wt % Class C fly ash, from about 0.1 wt. % to about 4 wt. % of lithium carbonate, from about 0.1 wt. % to about 5 wt. % potassium carbonate, and from about 0.1 to about 26% by weight of a retarding agent.
In yet another embodiment of the invention, a concrete mix is provided comprising stone, sand, water, and Portland cement, from about 25 to 65 wt % Class C fly ash, from about 0.1 wt. % to about 4 wt. % of lithium carbonate, from about 0.1 wt. % to about 5 wt. % potassium carbonate, and from about 0.1 to about 26% by weight of a retarding agent.
Accordingly, it is a feature of the present invention to provide a fly ash composition and concrete mix including a blend of fly ash, lithium carbonate, and potassium carbonate which results in reduced alkali-silica reactivity. Other objects and advantages of the present invention will become apparent from the following detailed description and the appended claims.